Complete guide to hot dip galvanizing for sheet metal parts. Learn about the process, coating thickness, corrosion resistance, and design considerations.
Introduction
Hot dip galvanizing provides exceptional corrosion protection for steel parts. At Fulei Metal, we offer galvanizing as part of our comprehensive surface treatment capabilities for outdoor and corrosive environment applications.
What is Hot Dip Galvanizing?
Steel is immersed in molten zinc at approximately 450 degrees C. The zinc reacts with the steel to form metallurgically bonded zinc-iron alloy layers. The coating is part of the steel surface, not just a surface layer.
Advantages
Long-lasting corrosion protection (20-50+ years). Metallurgical bond, no peeling or chipping. Protects all surfaces, including internal and recessed areas. Self-healing at cut edges (cathodic protection). Low maintenance. Cost-effective over service life.
The Galvanizing Process
Surface Preparation
Degreasing: remove oils and organic contaminants. Acid pickling: remove rust and scale. Fluxing: prevent re-oxidation before galvanizing. Surface must be clean and chemically active.
Galvanizing
Immersion in molten zinc at 445-465 degrees C. Dwell time depends on thickness: 2-10 minutes. Withdrawal rate affects coating thickness and appearance. Draining to remove excess zinc.
Post-Treatment
Quenching in water (sometimes with passivation). Cooling to handling temperature. Inspection and quality control.
Coating Structure
Layer 1 (innermost): Gamma layer, 21-28% iron, hard and brittle. Layer 2: Delta layer, 7-12% iron, hard and wear-resistant. Layer 3: Zeta layer, 5-6% iron, moderately hard. Layer 4 (outermost): Eta layer, pure zinc, soft and ductile.
Total thickness typically 50-200 microns depending on steel thickness and composition.
Design for Galvanizing
Venting
Hollow sections must be vented. Prevent pressure buildup from trapped air. Prevent zinc explosion. Vent holes at lowest and highest points. Minimum hole size: 10 mm.
Drainage
Design parts so zinc drains properly. Avoid pockets and cupped areas. Add drain holes where needed. Consider hanging orientation.
Material Selection
Silicon content affects coating thickness. Low silicon (under 0.02%): normal coating. High silicon (0.03-0.12%): thick, brittle coating. Very high silicon (over 0.12%): thick coating. Specify silicon-killed steel for consistent results.
Welding Considerations
Welding creates silicon-rich zones. May cause locally thick coating. Remove weld slag before galvanizing. Use compatible welding consumables.
Coating Thickness and Weight
| Steel Thickness | Coating Thickness | Coating Weight |
|---|
| 1-3 mm | 45-75 microns | 320-530 g/m2 |
| 3-5 mm | 60-100 microns | 420-700 g/m2 |
| 5-8 mm | 75-120 microns | 530-840 g/m2 |
| 8+ mm | 100-200 microns | 700-1400 g/m2 |
Corrosion Resistance
Atmospheric: 20-50+ years in rural, 10-30 years in urban, 5-15 years in industrial. Marine: 10-30 years depending on proximity to coast. Soil: varies widely with pH and moisture. Water: varies with temperature and chemistry.
Standards
ISO 1461: minimum coating thickness requirements. ASTM A123: American standard for structural steel. ASTM A153: for hardware and small parts. EN ISO 14713: guidelines for corrosion protection.
Common Issues
Zinc drips: excess zinc at drainage points. Rough surface: from fast withdrawal or high silicon. Bare spots: from poor surface preparation. Dark spots: from silicon-rich welds or steel. Warpage: from thermal stress in asymmetric parts.
Applications
At Fulei Metal, we galvanize: outdoor equipment, agricultural machinery, structural components, fencing, livestock equipment, and infrastructure components.
Conclusion
Hot dip galvanizing provides long-lasting, reliable corrosion protection. At Fulei Metal, our galvanizing capabilities and design support ensure properly galvanized parts for outdoor and corrosive applications.